In a manufacturing process of a flat panel display, a solar cell, a semiconductor device or the like, plasma is used for thin film formation and/or etching. Plasma is generated, for example, by introducing a gas into a processing container and applying a high frequency wave of several MHz to several hundred MHz to an electrode provided in the processing container. In order to improve the productivity, the size of a silicon substrate used for manufacturing a semiconductor device tends to become large year by year. Semiconductor devices have already been mass-produced with a silicon substrate having a diameter of 300 mm.
In plasma processes such as thin film formation and/or etching, it is required to generate plasma having a higher density in order to shorten the process time and improve the productivity. In addition, it is required to generate plasma having a low electron temperature in order to lower the energy of ions incident on a substrate surface to reduce ion irradiation damage and in order to suppress excessive dissociation of gas molecules. Generally, if the plasma excitation frequency is increased, the plasma density increases, and the electron temperature decreases. Therefore, it is desirable to increase the plasma excitation frequency in order to form a high-quality thin film with a high throughput. From such a viewpoint, a high frequency wave in a VHF (Very High Frequency) band of 30 to 300 MHz, which is higher than the frequency of an ordinary high frequency power supply of 13.56 MHz, is used for plasma processing (see, for example, Patent Documents 1 and 2).